Storage system and a method of speeding up writing data into the storage system

ABSTRACT

A storage system having disk drives, a first and a second cache memory for temporarily storing data sent from a host system so as to be written in the disk drives and a duplicate of the data, a first FIFO buffer for temporality storing the duplicate data in order to transfer the duplicate data from the first cache memory to the second cache memory, and a second FIFO buffer for temporality storing the duplicate data in order to transfer the duplicate data from the second cache memory to the first cache memory. In the case where the data sent from the host system so as to be written in the disk drives are temporarily stored in the first cache memory and the duplicate of the data is stored in the second cache memory, completions of the data writing are reported to the host system at the time point when the data and the duplicate data are stored in the fist cache memory and the second cache memory. After the report of completions of data writing to the host system, the duplicate data stored in the FIFO buffer are stored in the cache memory.

BACKGROUND OF THE INVENTION

The present invention relates to a storage system having a plurality of storage devices such as disk drives and, particularly, to a control technique for speeding up writing data sent from a host system such as a computer.

In a typical storage system having a plurality of storage devices, high speed data reading and data writing have heretofore been achieved by temporarily storing data sent and received between a host system and the storage devices in a cache memory such as a semiconductor memory. For example, in the case of writing data sent from the host system, the storage system temporarily stores the data in the cache memory and, at this time point, informs the host system about a completion of the data writing. After that, the storage system actually writes the data stored in the cache memory in the storage devices independently of operation of the host system.

In the case where a speed of sending and receiving data in the storage system is increased by the use of the cache memory as described above, the storage system informs the host system about the completion of data writing when the data is temporarily written in the cache memory though the data has not been written in the storage devices. Therefore, if the data in the cache memory are lost before the data are written in the storage devices due to a failure in the cache memory or the like, it is unable to recover the data. In view of this problem, JP-A-2001-318766 and JP-A-9-146842 disclose a technique of duplicating a cache memory (or a control unit including the cache memory) of a storage system and writing data in both of the cache memories for the purposes of reducing probability of the data loss and improving reliability of the storage system.

Also, a technique of preventing the data loss by the use of a nonvolatile cache memory or a more redundant nonvolatile cache memory is disclosed on pages 88 to 89 in IBM Total Storage Enterprise Storage Server Model 800 (IBM Redbooks, SG24-6424-01, Second Edition (October 2002), IBM Corp., ISBN 0738428256).

SUMMARY OF THE INVENTION

The storage systems disclosed in JP-A-2001-318766 and JP-A-9-146842 has limitation in reducing response time because of the time consuming process of writing data in the duplex cache memory and then reporting the host system about the completion of writing. More specifically, since one of the control units serving to perform data transfer between the host system and the storage system transfers data to the cache memory of the other control unit and receives the report about completion of the data writing in the other control unit, it is necessary for the duplex control unit to communicate with each other, and such communication is a main factor of prolonging the time required for the storage system to responsed to the host system. In general, in the duplex system, one system and the other system are made independent from each other by using separate power units or the like in order to avoid mutual influence which otherwise occurs in case of failure. Accordingly, the communication between one system and the other system is time consuming, and it is difficult to speeding up the communication.

In IBM Total Storage Enterprise Storage Server Model 800, since the nonvolatile memory is provided in one of the control units as a backup memory for a cache memory, the above problem, i.e., the delay due to the data transfer between the duplex system does not occur. However, in the case where a battery is used as a power unit for the nonvolatile memory, it is generally impossible to enlarge the size of storage capacity of the nonvolatile memory to be that of the cache memory because capacity of the battery must also be enlarged when enlarging the nonvolatile memory capacity.

Therefore, it is necessary to limit the quantity of the data to be written in the cache memory to that storable in the nonvolatile memory. Thus, it is still difficult to sufficiently enhance data writing performance of the storage system from the standpoint of the host system. That is to say, when the quantity of data to be written exceeds that storable in the nonvolatile memory, the storage system cannot accept data subsequently sent from the host system until the data written in the cache memory is written in the storage devices, thereby resulting in largely deteriorated performance.

An object of the present invention is to speed up writing data sent from a host system in a storage system when the storage system has duplex cache memory for the purpose of improving its reliability.

In order to solve the above problems, one embodiment of the storage system of the present invention has the following constitution. The storage system has a plurality of control units and storage devices. Each of the control units has a first memory and a second memory. The second memory can be a FIFO buffer having capacity less than that of the first memory. In the above constitution, one of the control units of the storage system that has received a request for writing data from a host system stores data corresponding to the writing request in the first memory and the second memory. At this time point, the host system is informed about a completion of the data writing. After that, the control unit transfers the data stored in the second memory to the first memory of the other control unit.

Further, the storage system has a plurality of power units for independently feeding power to the control units. The second memory of each of the control units is fed power by a power unit other than the power units feeding power to the control unit having the second memory.

Alternatively, the second memory may have a battery in addition to be fed by the power unit that feeds power to the control unit, so that the power source is switched to the battery in the case where the power from the power unit is interrupted.

Further, in order to connect the plurality of control units and the host system, the storage system may have switches and interface units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first example of embodiments of a storage system;

FIG. 2 shows one configuration example of a FIFO buffer used in the storage system;

FIG. 3 shows a third example of the embodiments of the storage system according to the present invention;

FIG. 4 shows one configuration example of a FIFO buffer with buttery used in the storage system;

FIG. 5 shows one example of an appearance of a mother board of a controller used in the storage system;

FIG. 6 shows one embodiment of a method of speeding up writing data into the storage system; and

FIG. 7 shows a fourth example of the embodiments of the storage system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.

FIG. 1 is a diagram showing a first embodiment of a storage system to which the present invention is applied. The storage system has two control units (hereinafter referred to as controllers) 10, two power units 11, and a plurality of storage devices (hereinafter referred to as disk drives) 12. As used herein, the disk drives 12 are devices obtainable by using a storage such as a hard disk and an optical disk. Each of the disk drives 12 is connected to both of the controllers 10. The power units 11 a and 11 b are independent from each other, wherein the power unit 11 a feeds power to the controller 10 a and the power unit 11 b feeds power to the controller 10 b. Thus, if one of the power units 11 is at fault, the storage system can continue its operation by feeding power to the controllers 10 by the use of the other one of the power units 11.

Each of the controllers 10 has a host interface unit 100, a FIFO buffer 102, a write completion control unit 103, and a cache memory 101. Further, the host interface unit and the write completion control unit may be realized by the combination of a processor and software or by dedicated hardware.

The storage system of this embodiment has a structure of duplex system. The cache memories 101 are volatile storages used for temporarily storing data sent and received between the disk drives 12 and a host system.

The host interface unit 100 a, the cache memory 101 a, and the write completion control unit 103 a of the controller 10 a are connected to the power unit 11 a via a power line 120 a. In turn, the host interface unit 100 b, the cache memory 101 b, and the write completion control unit 103 b of the controller 10 b are connected to the power unit 11 b via a power line 120 b.

The FIFO buffer 102 a of the controller 10 a is connected to the power unit 11 b via a power line 121 b. The FIFO buffer 102 b of the controller 10 b is connected to the power unit 11 a via a power line 121 a. By the above-described power line connection, the FIFO buffer 102 a of the controller 10 a is connected to the independent power unit different from that of other components of the controller 10 a, thereby establishing the duplex system of the FIFO buffer 102 a and the cache memory 101 a of the controller 10 a. Accordingly, if one of the power units 11 is at fault, the components (cache memories 101 or FIFO buffers 102) receiving the power feeding from the other one of the power units 11 can retain data. In the same manner, the duplex system of the FIFO buffer 102 b of the controller 10 b and the cache memory 101 b of the controller 10 b is established.

Hereinafter, a data writing process in the storage system of FIG. 1 will be described briefly. For example, when data sent from the host system connected to the controller 10 a are to be written into the storage system, the data is received first by the host interface unit 100 a. Note that the similar processing is performed when the data is received by the controller 10 b, too.

The data received by the host interface unit 10 a are then sent to the FIFO buffer 102 a and the cache memory 101 a via a signal line 110 a to be written therein. When the data writing is completed in the FIFO buffer 102 a without any failure, the FIFO buffer 102 a informs the write completion control unit 103 a about the completion of data writing via a signal line 111 a.

When the data writing is completed in the cache memory 101 a without any failure, the cache memory 101 a informs the write completion control unit 103 a about the completion of data writing via a signal line 112 a. Upon receipt of the information about the completions of data writing via the signal line 111 a and the signal line 112 a, the write completion control unit 103 a informs the host interface unit 100 a about the completions of data writing via a signal line 113 a. Upon receipt of the information, the host interface unit 100 a informs the host system about the completions of data writing. Here, since no communication is conducted between the controller 10 a and the controller 10 b, it is possible to speed up the data writing process in the storage system from the standpoint of the host system.

The data stored in the FIFO buffer 102 a is transferred to and written in the cache memory 101 b of the controller 10 b via a signal line 114 a after the host interface unit 100 a has informed the host system about the completions of data writing. Thus, the communication relating to the data transfer is conducted between the controller 10 a and the controller 10 b after the completions of data writing are reported to the host system. Further, since the data stored in the FIFO buffer 102 a are transferred to the cache memory 101 b, it is possible to clear the FIFO buffer 102 a for writing data subsequently sent from the host system. Accordingly, because a situation wherein the data sent subsequent from the host system have not been written in the storage system until the data are written in the disk drives 12 due to a lack of capacity in the FIFO buffer 102 a does not occur, it is possible to further enhance performance of the data writing process.

FIG. 2 is a diagram showing one configuration example of the FIFO buffer 102 a. The FIFO buffer 102 b has the same configuration as that of the FIFO buffer 102 a except that the power units 11 a and 11 b are exchanged. The FIFO buffer 102 a is a memory having a power supervising unit 301 a, a write controller 302 a, a read controller 303 a, a FIFO memory 304 a, a data checker 305 a, and a data remaining indicator 330 a.

By the use of a CRC or the like, the data checker 305 a checks whether or not the data sent from the host system are erroneous due to failure that might have occurred during the transfer. When the data checker 305 a confirms that there is no error, the result is sent to the write controller 302 a via a signal line 311 a. Upon receipt of the result from the data checker 305 a, the write controller 302 a outputs a write signal via a signal line 313 a. Upon receipt of the write signal, the FIFO memory 304 a stores the data transferred from the host system.

Upon completion of the data writing, the FIFO memory 304 a informs the write controller 302 a about the completion of data writing via the signal line 313 a. Upon receipt of the completion of data writing, the write controller 302 a informs the write completion control unit 103 a about the completion of data writing via the signal line 111 a. The write controller 302 a also informs the read controller 303 a about the completion of data writing in the FIFO memory 304 a via a signal line 315 a.

After being informed about the data writing in the FIFO memory 304 a via the signal line 315 a, the read controller 303 a outputs a read signal via a signal line 314 a. Upon receipt of the read signal, the FIFO memory 304 a outputs the stored data to the signal line 114 a, so that the data is transferred to the cache memory 101 b of the controller 10 b.

The power supervising unit 301 a supervises states of the power unit 11 a using a supervising signal line 310 a (not shown in FIG. 1). The reason for the supervision of the power unit 11 a is as follows. Since the signal from the host interface unit 100 a is lost in the case where a failure of the power unit 11 a occurs to result in a stoppage of power feeding, the FIFO buffer 102 a can detect the failure in the power unit 11 a only from presence or absence of the signal from the host interface unit 100 a. However, if the power unit 11 a feeds an erroneous voltage which is not a correct voltage, it is expected that the signal from the host interface unit 100 a is not simply lost but causes erroneous operation that leads to generation of an abnormal signal. In this case, supervision of the signal from the host interface unit 110 a is not sufficient for detecting such abnormal signal. Therefore, the power supervising unit 301 a directly supervises the power unit 11 a in order to judge whether or not the signal from the host interface unit 100 a is expected to be normal.

If a failure occurs in the power unit 11 a to bring about a state wherein data are not sent from the host interface unit 10 a normally, the power supervising unit 301 a outputs a write inhibition signal via a signal line 312 a to the write controller 302 a so as to inhibit the erroneous data from being stored in the FIFO memory 304 a. Even if a failure occurs in the power unit 11 a, components included in the FIFO buffer 102 a can continue their operation without any failure since they are connected to the power unit 11 b via a power line 320 a and the power line 121 b.

The FIFO buffer 102 a is provided also with a data remaining indicator 330 a for displaying whether or not data that have not been transferred to the cache memory 101 b of the controller 10 b remain in the FIFO memory 304 a. In the case where a failure occurs in the controller 10 a and the failure is to be restored by exchanging the controller 10 a with a new one, a user or a manager of the storage system confirms whether or not the data transfer to the cache memory 101 b of the controller 10 b has been completed owing to the data remaining indicator 330 a and then restores the failure by exchanging the controllers 10 a after completion of the data transfer.

A storage system of the present invention may have the following second embodiment, which has power units 43 a and 43 b in addition to the constitution of the first embodiment. In the storage system, a FIFO buffer 402 a is connected to the power unit 43 b and a FIFO buffer 402 a is connected to the power unit 43 a. With the constitution of the second embodiment, the power units 43 a and 43 b serve only to feed power respectively to the FIFO buffers 102 a and 102 b; therefore, it is possible to reduce capacities of the power units as compared with those of the power units 11 a and 11 b.

FIG. 3 is a diagram showing a third embodiment of a storage system to which the present invention is applied. The storage system of the third embodiment is different from the first embodiment in that it has FIFO buffers 502 a and 502 b each of which includes a battery. Further, unlike the first embodiment, the FIFO buffer with battery 502 a is connected to a power unit 11 a via a power line 521 b, and the FIFO buffer with battery 502 b is connected to a power unit 11 b via a power line 521 a in this embodiment. The FIFO buffers with batteries 502 a and 502 b continue their operation by the use of the internal batteries in case of failures of the power units 11 a and 11 b. Other components and operation thereof are the same as those of the first embodiment.

FIG. 4 is a diagram showing one configuration example of the FIFO buffer with battery 502 a. Configuration of the FIFO buffer with battery 502 b is substantially the same as that of the FIFO buffer with battery 502 a except that the power unit 11 b is used in place of the power unit 11 a. The FIFO buffer with battery 502 a has a power supervising unit 301 a, a write controller 302 a, a read controller 303 a, a FIFO memory 304 a, a data checker 605 a, a data remaining indicator 330 a, a battery 606 a, a charge controller 607 a, and a power selector 608 a.

The power supervising unit 301 a supervises states of the power unit 11 a by the use of a supervising signal line 310 a (not shown in FIG. 3). Upon detection of a failure in the power unit 11 a via the supervising signal line 301 a, the power supervising unit 310 a outputs a write inhibition signal to the write controller 302 a via a signal line 312 a in the same manner as in the first embodiment, so that new data are inhibited from being stored in the FIFO memory 304 a. Also, in this embodiment, the power supervising unit 301 a outputs a power unit switch signal to the power selector 608 a via a signal line 616 a simultaneously with the output of the write inhibition signal. Upon receipt of the power switch signal, the power selector 608 a switches the power unit to be connected to a power line 620 a connected to all the components from the power unit 11 a to the battery 606 a.

The battery 606 a is charged by the charge controller 607 a as long as the power unit 11 a is in a normal state. Thus, the battery 606 a is always ready to feed power to all the components in place of the power unit 11 a. When the power switch signal is output from the power supervising unit 301 a, the battery 606 a feeds power to all the components of the FIFO buffer 502 a via the power line 620 a. Required duration of the power feeding from the battery 606 a is from the time point when the power is switched from the power unit 11 a to the battery 606 a until all the data stored in the FIFO memory 304 a are transferred to the cache memory 101 b of the controller 50 b. The duration is sufficient since no new data are stored in the FIFO memory after the power is switched to the battery 606 a due to the write inhibition signal output from the power supervising unit 301 a. Thus, the battery 606 a does not have to be of large capacity, and a battery having small capacity is sufficiently used as the battery 606 a.

Operation of the FIFO buffer with battery 502 a other than that describe above is the same as the FIFO buffer of the first embodiment.

FIG. 5 is a diagram showing a specific example of installation of the controllers 50 shown in FIG. 3. Mounted on a mother board 70 are connectors for host connection 71 connected to signal lines from a host system, an LSI 72 serving as a host interface unit, an LSI 74 serving as a write completion control unit, a cache memory 73 in the form of a DIMM (Dual In-line Memory Module), and a daughter card for FIFO buffer 75. Further, mounted on the daughter card for FIFO buffer 75 are an LSI 751 serving as a FIFO memory, an LSI 752 including a write controller, a read controller, a power supervising unit, a data checker, and a charge controller, and a battery 753.

The mother board 70 is connected to the other one of the controllers 50, disk drives 12, and the power units 11 a and 11 b via an edge connecting part 76. This means that power is fed to the components on the mother board 70 from the power unit 11 a or the power unit 11 b. Power is fed to the components on the daughter card for FIFO buffer 75 from the power unit 11 a, the power unit 11 b or the battery 753.

Referring to FIG. 5, a data remaining indicator 754 is mounted on the mother board 70 near a card edge at the side of the connectors for host connection 71 in the form of an LED. With such constitution, it is possible to observe the card edge at the side of the connectors for host connection 71 on the mother board 70 from outside when the controllers 50 are incorporated in the storage system. Thus, it is possible for the user or the manager of the storage system to observe the card edge from outside thereby to readily detect whether or not each of the controllers 50 is replaceable in case of failure.

Further, because the data remaining indicator 754 is mounted on the mother board 70, not on the daughter card for FIFO buffer 75, the data remaining indicator 754 and the daughter card for FIFO buffer 75 are connected so that power is fed to the data remaining indicator 754 via the signal line from the daughter card for FIFO buffer 75 when the power feeding from the power units 11 is stopped.

In the case where the remaining data indicator 754 is an LED, for example, the LED is adapted to emit light for indicating that each of the controllers 50 is replaceable and to be unlit for indicating that the controller 50 is not replaceable in order to prevent the LED from erroneously indicating that there is no remaining data and the controller 50 is replaceable though there is in fact data remaining in the FIFO memory. If the above indications are reversed, it is impossible for the user or the manager to distinguish the unlit state of the remaining data indicator 754 caused by disconnection of the signal line from the daughter card for FIFO buffer 75 due to some failure on the mother board 70 from the unlit state indicating that the controller 50 is replaceable.

However, it is possible to distinguish the unlit state caused by some failure on the mother board 70 from the unlit state indicating that the controller 50 is replaceable by the use of other means. For example, the maximum time required for transferring all the data from the FIFO memory to a cache memory of the other one of the controllers 50 may be measured in advance, so that an unlit state of the data remaining indicator 754 lasting over the maximum time can be recognized as that caused by some failure on the mother board 70.

Alternatively, in the case where it is possible to detect the data which have been sent last from the host system in the host system, the distinction may be attained by detecting in the other controller 50 whether or not the written data are stored in the cache memory or the disk drives of the other controller 50.

FIG. 6 is a flowchart showing a process of writing data into the storage system to which the present invention is applied. The flowchart is common to the foregoing embodiments.

The controller 10 a of the storage system receives a write command from the host system via the host interface unit 100 (Step 801). Then, the controller 10 a allocates a space in the cache memory 101 a for the size of data designated by the write command (Step 802). The controller 10 a then inspects in the FIFO buffer 102 a for the space (Steps 803 and 804), and, when there is no space, the controller 10 a carries out the processing of Steps 803 and 804 repeatedly until the space is created in the FIFO buffer 102 a.

In the case where there is the space or the space is created, the controller 10 a informs the host system about a completion of write preparation (Step 805). After that, the controller 10 a receives the data from the host system (Step 806). Upon receipt of the data, the host interface unit 100 a of the controller 10 a duplicates the received data. Here, another unit may be used for performing the data duplication (Step 807). The host interface unit 100 a then transfers one of the duplicated data to the cache memory 101 a and the other duplicated data to the FIFO buffer 102 a (Step 808).

Then, the controller 10 a inspects whether or not an error has occurred in writing the data in the cache memory 101 a and the FIFO buffer 102 a (Step 809). If any error has occurred in either one of the data writings, the controller 10 a informs the host system about the write error (Step 810). If no error has occurred in the data writings, the controller 10 a inspects whether or not receipt of the data sent from the host system has completed (Step 811). If the receipt has not completed yet, the controller 10 a repeats the processing of Steps 806 to 811 until all the data is received.

After the completion of receiving all the data, the controller 10 a informs the host system about the completion of data writing (Step 812). The controller 10 a instructs the controller 10 b of another system (for back-up) to allocate a space in the cache memory 101 b. More specifically, the FIFO buffer 102 a may output a write command signal directly to the controller 10 b (Step 813), for example. After the space has been allocated, the controller 10 a transfers the data from the FIFO buffer 102 a to the cache memory 101 b for back-up, thereby terminating the process (Step 814).

With such process steps, it is possible for the storage system to inform the host system about the completion of data writing before the data is stored in the cache memory for back-up.

FIG. 7 is a fourth embodiment of a storage system to which the present invention is applied. The storage system of this embodiment has four controllers 90 a, 90 b, 90 c, and 90 d. Among the controllers 90, the controllers 90 a and 90 b are of duplex system and the controllers 90 c and 90 d are of duplex system. That is, a power unit 11 a feeds power to the controllers 90 a and 90 c, and the power unit 11 b feeds power to the controllers 90 b and 90 d. The storage system of this embodiment has exchanging and switching units 905 and independent host interface units 900 in addition to the components of the storage system of the third embodiment.

Each of the controllers 90 has a connection switching unit 904 in place of the host interface unit 100 included in the controllers of the foregoing embodiments. The host interface units 900 and the connection switching units 904 of the controllers 90 are mutually connected via the exchanging and switching units 905. The host interface units 900 and the exchanging and switching units 905 may be replaced by a single host interface unit and a single exchanging and switching unit. With the above constitution, it is possible to use any one of the controllers 90 a, 90 b, 90 c, and 90 d in both of the case wherein the host system is connected to the host interface unit 900 a and the case wherein the host system is connected to the host interface unit 900 b.

In this embodiment, when writing data sent from the host system connected to the host interface unit 900 a into the storage system, for example, the data are transferred to the connection switching unit 904 a of the controller 90 a via, for example, the exchanging and switching unit 905 a. The data received by the connection switching unit 904 a are transferred to the FIFO buffer with battery 502 a and the cache memory 101 a via the signal line 110 a.

After completing the data writing without any failure, the FIFO buffer with battery 502 a informs the write completion control unit 103 a about the completion of data writing via the signal line 111 a. After completing the data writing without any failure, the cache memory 101 informs the write completion control unit 103 a about the completion of data writing via the signal line 112 a. After being informed of the completions of data writing via the signal lines 111 a and the 112 a, the write completion control unit 103 a informs the connection switching unit 904 a about the completions of data writing via the signal line 113 a. The connection switching unit 904 a informs the host system about the completions of data writing via the exchanging and switching unit 905 a and the host interface unit 900 a. Since no communication takes place between the controller 90 a and other controllers 90 b, 90 c, and 90 d for informing the data transfer, it is possible to speed up the writing process in the storage system from the standpoint of the host system.

The data stored in the FIFO buffer 502 a are transferred to the cache memory 101 b of the controller 90 b via the signal line 114 a after the host interface unit 900 a has informed the host system about the completions of data writing. Thus, the communication between the controller 90 a and the controller 90 b is performed after the report of the completions of data writing to the host system. Further, since the data stored in the FIFO buffer with battery 502 a are transferred to the cache memory 101 b, it is possible to clear the FIFO buffer with battery 502 a for writing data subsequently sent from the host system. Therefore, a situation wherein the subsequent data from the host system is not written until the completions of data writing on the disk drives 12 due to a lack of capacity in the FIFO buffer with battery 502 a does not occur. Thus, it is possible to enhance the performance of writing process in the storage system.

The operation of the FIFO buffers with batteries 502 of this embodiment is the same as that of the FIFO buffers with batteries 502 of the third embodiment.

According to the present invention, it is possible to reduce time required for data writing in the storage system having the duplex cache memory from the standpoint of the host system. Thus, it is possible to improve data writing performance from the standpoint of the host system. 

1. A storage system comprising: a storage device; and a first controller and a second controller both connected to the storage device and a host system, wherein the first controller has a first memory and a second memory, wherein, the second controller has a third memory, wherein, in the case where the fist controller receives data from the host system, the first controller stores the data in the first and the second memories and sends a response to the host system, and then the first controller transfers the data stored in the second memory to the third memory.
 2. A storage system according to claim 1, further comprising a first power unit and a second power unit, wherein the first memory of the first controller receives power feeding from the first power unit, wherein the second memory of the first controller and the third memory of the second controller receive power feeding from the second power unit.
 3. A storage system according to claim 1, further comprising a first, a second and a third power unit, wherein the first memory receives power feeding from the first power unit, wherein the second memory receives power feeding from the second power unit, wherein the third memory receives power feeding from the third power unit.
 4. A storage system according to claim 1, further comprising a power unit, wherein the first memory and the second memory receive power feeding from the power unit, wherein the second memory includes a battery and charges the battery by the use of the power unit.
 5. A storage system according to claim 4, wherein, in the case where a failure occurs in the power unit, the second memory switches the power feeding from the power unit to power feeding from the battery.
 6. A storage system according to claim 2, wherein the second memory is a FIFO buffer.
 7. A storage system according to claim 6, wherein the storage device is a plurality of storage devices.
 8. A storage system according to claim 7, wherein the second memory has a unit for indicating presence or absence of data stored in the second memory.
 9. A storage system comprising: a host interface unit connected to a host system; a switching unit connected to the host interface unit; a first and a second controller connected to the switching unit; and a storage device connected to the first and the second controller, wherein the first controller has a first memory and a second memory, wherein the second controller has a third memory, wherein, in the case where the first controller receives data from the host system, the first controller stores the data in the first and the second memory and sends a response to the host system, and then the first controller transfers the data stored in the second memory to the third memory.
 10. A storage system comprising: a disk drive; a first cache memory for temporarily storing data sent from a host system so as to be written on the disk drive; a second cache memory for storing a duplicate of the data to be written on the disk drive; and a FIFO buffer for temporarily storing the duplicate of the data sent from the host system to transfer the duplicate of the data to the second cache memory, wherein, at the time when the data sent from the host system are stored in the first cache memory and the duplicate of the data sent from the host system are stored in the FIFO buffer, the host system is informed about the completions of data writing.
 11. A storage system according to claim 10, further comprising: a first power unit connected to the first cache memory; and a second power unit connected to the second cache memory, the second power unit being independent from the first power unit, wherein the FIFO buffer is connected to the second power unit.
 12. A storage system according to claim 10, further comprising: a first power unit connected the first cache memory; a second power unit connected to the second cache memory, the second power unit being independent from the first power unit; and a third power unit connected to the FIFO buffer, the third power unit being independent from the first power unit.
 13. A storage system according to claim 10, further comprising: a first power unit connected the first cache memory; and a second power unit connected to the second cache memory, the second power unit being independent from the first power unit, wherein the FIFO buffer is connected to the first power unit and provided with a battery for feeding power to the FIFO buffer in place of the first power unit in case of failure in the first power unit.
 14. A storage system according to claim 11, comprising a data remaining indicator for indicating whether or not all the duplicate of the data sent from the host system has been transferred from the FIFO buffer to the second cache memory.
 15. A method of writing data in a storage system having duplex cache memory, comprising the steps of: writing data in one of the duplex cache memory for duplicating and storing data sent from host system; writing the data in a FIFO buffer capable of performing writing with a speed higher than that of the other one of the duplex cache memory; confirming that the data have been correctly written in said one of the duplex cache memory and the FIFO buffer and informing the host system about completions of the data writing; and writing the data written in the FIFO buffer in the other one of the duplex cache memory after the host system is informed about the completions of data writing to complete duplication of the data. 